Total number and ratio of excitatory and inhibitory synapses converging onto single interneurons of different types in the CA1 area of the rat hippocampus

The least known aspect of the functional architecture of hippocampal microc ircuits is the quantitative distribution of synaptic inputs of identified c ell classes. The complete dendritic trees of functionally distinct interneu ron types containing parvalbumin (PV), calbindin D-28k (CB), or calretinin (CR) were reconstructed at the light microscopic level to describe their ge ometry, total length, and laminar distribution. Serial electron microscopic reconstruction and postembedding GABA immunostaining was then used to dete rmine the density of GABA-negative asymmetrical (excitatory) and GABA-posit ive symmetrical (inhibitory) synaptic inputs on their dendrites, somata, an d axon initial segments. The total convergence and the distribution of exci tatory and inhibitory inputs were then calculated using the light and elect ron microscopic data sets. The three populations showed characteristic differences in dendritic morpho logy and in the density and distribution of afferent synapses. PV cells pos sessed the most extensive dendritic tree (4300 mu m) and the thickest dendr ites. CR cells had the smallest dendritic tree (2500 mu m) and the thinnest shafts. The density of inputs as well as the total number of excitatory pl us inhibitory synapses was several times higher on PV cells (on average, 16 ,294) than on CB (3839) or CR (2186) cells. The ratio of GABAergic inputs w as significantly higher on CB (29.4%) and CR (20.71%) cells than on PV cell s (6.4%). The density of inhibitory terminals was higher in the perisomatic region than on the distal dendrites. These anatomical data are essential to understand the distinct behavior and role of these interneuron types during hippocampal activity patterns and r epresent fundamental information for modeling studies.